Connection Inhibition Apparatus and Method

ABSTRACT

The apparatus transmits in a first zone, for example, an aircraft, a masking signal which masks transmissions from a second zone outside of the first. By doing this a mobile cellular telephone is inhibited from connecting or attempting to connect to base-stations on the ground. A hole in the masking signal spectrum may be provided to enable connection to a base-station within the aircraft. Alternatively, a base-station within the aircraft may be arranged to transmit at a power level greater than the masking signal.

BACKGROUND

This invention relates to apparatus for inhibiting connection ofcommunication equipment to communication infrastructure.

SUMMARY

It is undesirable for mobiles to be used in particular circumstances.The signals emanating from the mobile are considered by some to bepotentially interfering with, for example, the aircraft flight systemsor other sensitive equipment. Another issue is that the mobile may, froma high flying aircraft, seek to interact with a number of base-sitesserving disparate geographical areas.

It is therefore desirable to provide an on-board local cell site toserve the mobiles in such a way that those mobiles are controlled tooperate using minimum transmitted power. This may also serve as anadditional revenue stream for the aircraft operator as well aspreventing interference to ground based infrastructure.

When switched on, mobiles scan the available frequencies to “find” abase-station. Thus, it is possible for a mobile to encounter a groundbased base-station before encountering an on-board base-station. It isdesirable to prevent this.

Accordingly, in a first aspect the invention relates to connectioninhibition apparatus to prevent a mobile communication apparatus in afirst zone connecting to a base station in a second zone outside of thefirst which connection inhibition apparatus comprising means to transmita masking signal at the frequency or frequencies of the base-stationwithin the first zone to mask transmissions from the second zone toinhibit connection thereto.

In the case of an aircraft, the first zone would be the flight cabin andthe second zone will be outside of the aircraft.

The inventors have also appreciated that the transmitted masking signalwill be subject to multipath Rayleigh fading within the first zone.Thus, there will be locations within the first zone where destructiveinterference results in deep fading of the masking signal. In theselocations a mobile may be able to detect base-stations outside of thefirst zone because the masking signal generated in the first zone willbe insufficient to mask the base-stations in the second zone.

In attempting to establish connection to the base-station in the secondzone the mobile will transmit a RACH (random access channel) at or nearto full power. This high power signal may be considered undesirable in,for example, an aircraft cabin. It is also undesirable because it willgenerate interference in base stations other than the one to which it isattempting to affiliate.

Thus, in accordance with a second aspect of the invention connectioninhibition apparatus comprises means to transmit a masking signal whichtransmits two or more signals which are mutually non-coherent. Byproviding two or more mutually non-coherent masking signals fromseparate antennas or antenna systems the problem of deep fading isalleviated or eliminated because the two or more sources of maskingsignal will be received over independent channels at the mobile receiverand will sum according to their independent powers in such a way that adeep fade will be experienced only in the unlikely event that allsources independently experience deep fading. Preferably, the apparatusincludes in the first zone a first zone base-station to which the mobilecommunication equipment may establish connection.

In accordance with another aspect of the invention, means to transmit amasking signal transmits a spectrum with a discontinuity or notch atthat part of the spectrum to be used by the first zone base-station. Thepreferred masking signal is a chirped waveform.

By doing this the masking signal will not mask the first zonebase-station.

Preferably, the masking signal is a phase modulated by a randomisingsequence and the preferred sequence is an M sequence.

According to a yet further aspect of the invention there is provided acoupling network for coupling a first and a second masking signal from afirst masking signal source, a third masking signal from a secondmasking signal source and a transmitter signal to a first and a secondantenna comprising; means to couple the first and the third maskingsignals to respective ones of the first and the second antenna, means tocombine the transmitter signal and the second masking signal to providea combined signal, and means to couple the combined signal to bothantennas. This coupling network may be combined with the earlier aspectsof the invention.

A yet further aspect of the invention provides a method for producing anotched chirped waveform for use as a masking signal in apparatus as inthe earlier aspects comprising the steps of; providing a chirpedwaveform,

identifying a part of the chirped waveform where a notch is required,applying a transform to the identified part to provide spectralcomponents, deriving from the spectral components those that are to benulled, applying an inverse transform to the spectral components to benulled, and subtracting the inverse transformed spectral components fromthe chirped waveform to generate the notched chirped waveform.

Preferably, the chirped waveform is held in memory.

Preferably, the spectrum of the chirped waveform is also held in memoryas a set of complex weights and further comprising the steps of;selecting from the set of complex weights a selection applicable to adesired notch, deriving from the selection a set of waveforms, andapplying the set of waveforms to the chirped waveform to generate anotched chirped waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

A specific embodiment of the invention will now be described, by way ofexample only, with reference to, and as illustrated by, the drawings inwhich:

FIG. 1 shows in block diagram form apparatus in accordance with theinvention being used in an aircraft cabin;

FIG. 2 shows more detail of the chirped waveform generator of theapparatus of FIG. 1;

FIG. 3 shows a ramp waveform used in the generation of a chirpedwaveform;

FIG. 4 shows an M sequence;

FIGS. 5 to 8 are explanatory figures concerning the generation of amasking signal;

FIG. 9 shows an alternative embodiment of the apparatus in which amasking signal is held in a memory structure in the form of look-uptables;

FIG. 10 shows a yet further embodiment of the apparatus in which alocation of the zone is determined and used to select and apply anappropriate masking signal scheme;

FIG. 11 is an explanatory figure showing the use of the masking signalsto cater for the masking of different bands; and

FIGS. 12 to 14 show various embodiments of coupling networks for use inthe apparatus.

DETAILED DESCRIPTION OF EMBODIMENTS

As is shown in FIG. 1 connection inhibition apparatus 1 comprises twomasking signal generators 2 a, 2 b connected to antennas 3 and 4 formedby leaky feeders by a coupler network 2 c located in an aircraft cabin5.

The apparatus 1 is coupled to a base-station 6. The base-station 6 iscoupled to both antenna 3 and 4 by the coupler network 2 c. Thebase-station 6 is also connected via a base station controller function,to a further antenna 8 mounted on the exterior of the aircraft. Thebase-station 6 serves a cell formed by a first zone which is the cabin5. Mobiles 9 connect to the base-station 6 and thus by the antenna 8 toa ground based communications network. This connection to the groundbased communication network will typically be directed via a satellitecommunications link, wherein antenna 8 is a satellite antenna.

One or more further ground based mobile communications networks (theseare not the same ground based communications networks as mentioned inthe previous paragraphs) include a number of base-stations 10 to 12serving different geographic areas on a cellular basis. The area outsideof the cabin is referred to in this description as the second zone.

The chirped waveform generator may take a number of forms. In thespecific embodiment it relies upon a number of stored values for thechirped waveform which are accessed and used to drive appropriatecircuitry. Alternatively, the chirped waveform may be directly generatedand used. For an understanding of the preferred embodiment, the directlygenerated alternative will be first described with reference to FIG. 2.

As is shown in FIG. 2, the chirped waveform generator 13 comprises aclock 131 having a period of 5 microseconds feeding a clock signal to achirp ramp generator 132 of a period of again 5 microseconds. The outputramp is applied to modulate a sinusoidal signal produced by a voltagecontrolled oscillator 133. The voltage controlled oscillator 133 has acentre frequency of 1842.5 MHz. This produces an output chirped waveform134 which may be applied to the transmitter sections.

However, in an enhancement to this basic embodiment, which provides achirped waveform that is preferred, additional circuitry is provided.This is represented in the figure by the broken outline 135. This takesthe clock signal to a randomising sequence generator 136 which in thepreferred form is an M sequence generator. This is used to modulate thechirped waveform 134 to provide an M sequence modulated chirped waveformvia mixer 137 which is applied to the transmitter sections.

The ramp generated waveform is shown in more detail in FIG. 3 and it canbe seen that it is a sawtooth waveform with a rapid flyback period a.This establishes in conjunction with the VCO, the flyback period of theresultant chirped waveform 134.

FIG. 4 shows the M sequence generator produce a binary waveform ofconstant amplitude. The M sequence is also known as a shift registersequence and it is a repeating pattern of length 2̂n−1. The clock 131clocks the M sequence generator 136 such that in the period of onechirp, one M sequence element is produced. The effect of this when mixedat the mixer 137 with the chirped waveform is that the chirped waveformis either inverted or not inverted for a single period of the chirpedwaveform. Thus, the period of an element of the binary sequence is,preferably, an integer multiple of the period of a chirp used to formthe chirped waveform. It is also preferred that the time of transitionof one element of the binary sequence to the next is arrangedsubstantially to coincide with the flyback period of the chirpedwaveform. The effect of the M modulation is to biphase modulate thechirped waveform. It produces a train of waveforms which avoid deepfading within the first zone by ensuring diversity through non-coherentcombining between the outputs of two or more independent generatorsreceived over separate radio paths.

In a further preferred enhancement to the chirp waveform generator anamplitude windowing function is applied by a windowing means 138 shownin broken outline. This is an enhancement applicable to bothalternatives.

FIG. 5 shows the spectrum of a chirped waveform and FIG. 6 a windowingfunction to be applied to the chirped waveform by the windowing means138. This windowing function has a raised cosine time domain functionwith a parameter k determining the proportion of the window that followsthe raised cosine function. In the illustrated example k=0.4 but otherwindow functions may be used. FIG. 5 has a vertical axis of relativelevel in dB and the horizontal axis is frequency running from −50 to +50MHz relative to the centre frequency. The nominal bandwidth of thesignal is ±10 MHz. When the window is applied to the waveform of FIG. 5it produces a relatively flat central region (−10 to +10 MHz) andcreates a relatively fast roll off of the signal spectrum outside of thecentral portion to provide the windowed chirped spectrum of FIG. 7 (Inthis case it relates to a windowed chirp where k=0.1). It will be seenthat a comparison of the central portions b of the waveforms in FIGS. 5and 7 show that the flatness is improved. Observing the differences inthe scales of the vertical axis in FIGS. 5 and 7, it will be alsonoticed that the roll-off over the regions c and d is more rapid. Thesignal is no longer has a constant envelope but the peak to mean ratiois only increased to 0.28 dB.

The chirped waveform and the windowed chirped waveform whose spectra areillustrated in FIGS. 5 and 7 respectively may be used in embodimentswhich are required merely to inhibit connection of equipment in thefirst zone to base-stations in the second. However, in a furtherpreferred enhancement the chirped waveform or the windowed chirpedwaveform is generated with a hole or discontinuity in its profile whichsubstantially co-incides with the operating frequency of thebase-station serving the first zone, that is to say, the base-station 6of FIG. 1. This is illustrated by FIG. 8 which shows a windowed chirpedwaveform with a notch, discontinuity or hole e which co-incides with theoperating band of the base-station 6 at an example frequency of about +6MHz relative to the centre frequency In essence therefore, the waveformwill mask or jam the signals across the communications system band fexcept those within the notch e.

Notch generation can be done by taking a Fourier transform of thechirped waveform, nulling the frequencies where the hole is required andperforming an inverse Fourier transform of the result. This gives riseto a chirp with a notch as shown in FIG. 8. This notched chirpedwaveform is used in a later embodiment of the apparatus using values forthe waveform held in look-up tables.

Ideally, the hole will have a depth commensurate with the precision ofthe quantisation used in the sample. However, with a power amplifierwhich will be non-ideal, intermodulation products will tend to “in-fill”the hole. Thus in FIG. 9, the hole has a null of depth 40 dB when theamplifier has a 6 dB back off. The peak to mean ratio is about 1.25 dB.

It will be appreciated that more than one hole, notch or discontinuitymay be provided depending on the requirements of the base-station orbase-stations serving the zone.

In summary a number of alternative masking signals may be generated byvarious alternative embodiments of the invention.

In a first masking signal a chirped waveform is applied to thetransmitter section.

In a second masking signal a chirped waveform is windowed to provide awindowed chirped waveform to be applied to the transmitter section.

A third or fourth masking signal is provided by providing a notchedversion of the first and second.

A yet further set of types is where the earlier types are modulated by arandomising sequence which in its preferred form is an M sequence. Thisensures signal diversity to prevent deep fading within the first zone ofthe masking signals.

In the preferred embodiment, the masking signal, whatever its type,whether the chirped waveform, the windowed chirped waveform or thewindowed chirped and notched waveform is pre-loaded into look-up tablesheld in memory and the values output to generate the waveforms. Thisembodiment will be described with reference to FIG. 9. It comprises asquare wave signal generator 17 feeding a counter 18. The count value isinput to look-up tables 19 and 20 in the respective phase and quadraturesignal branches.

The look-up tables 19 and 20 hold a representation in coded form of amasking signal and output the relevant part of the look-up tables inresponse to the count from the counter 18. The relevant part is thenpassed to respect digital to analogue converters 21 and 22. The look-uptables are populated with data by use of the lines 19 a and 20 a. Thedata may be loaded at configuration of the equipment or when it is inuse in an adaptive manner.

The resultant analogue signal is passed via low pass filters 23 and 24to respective mixers 25, 26.

At the mixers 25, 26 the signals are mixed with an RF signal provided bya signal generator 27. The in phase and quadrature signals are combinedby combiner 28 before being output to the transmitter sections 14 and15.

The coded representations of the signal held in the look up tables forthe two chirp generators differ according either to the phase orgenerator polynominal used for the M sequence. This is to ensure thatthe resultant signals transmitted in to the cabin are non-coherent toavoid the above mentioned fading problem

Thus, when the mobile is switched on in the cabin 5 (zone 1) it scansthe bandwidth b of the earlier described chirped windowed and notchedwaveform. Signals emanating from the ground based base-stations 10 to 12are not distinguishable from the masking signal. Scanning the bandwidththe mobile will arrive at the transmissions at the frequency of thenotch e emanating from the base-station 6. Following the normalinitialisation functions of the mobile phone, having found only onevalid mobile phone downlink signal, a connection will then beestablished.

Diversity of the output masking signals is ensured by the use of the Msequence modulation of the stored waveform wherein the M sequencegenerator polynominals or code phases across two or more masking signalgenerators are different In this way, any alignment of carrier phase onthe individual masking signal outputs as received over separate radiopaths from separate transmitting antenna at a mobile antenna will bemodulated so that interference is sometimes constructive, sometimesdestructive, resulting in an effective power-wise addition of thesignals.

It will be appreciated that in the case of mobile cellulartelecommunications there are a number of standards that are applied indifferent parts of the world. It is possible that passengers in theaircraft cabin may have mobiles which operate according to differentstandards. It may be necessary to provide masking signals which mask thetransmissions according to the different standards. Furthermore, it maybe desirable to apply different masking signals as the aircraft travelsand crosses different geographical areas. For example, as an aircraftflying from Europe enters range of a mobile telecommunications systemoperating in the USA.

FIG. 10 shows in block diagram form an embodiment which determines alocation of the aircraft and from that determined location selects aparticular masking signal scheme from memory which is then applied. Itis a modification to the apparatus shown in FIG. 9 where the values heldin lookup tables 19,20 are manipulated or selected dependent upon adetermined location. The apparatus includes a global positioningreceiver 110 which determines a current location of the aircraft andpasses that location to a processor 112. The processor 112 accessesmemory 113 which holds a set of masking schemes and selects a maskingscheme appropriate for the determined location and wherein the frequencyof the oscillator 27 may also be selected dependent upon that determinedlocation. The masking scheme is then downloaded to the lookup tables 19,20 and applied as before. (Alternatively, the lookup tables may hold anumber of schemes and the scheme to be operated may be selected by theprocessor 112.)

For some less complex arrangements, the notch in the chirped waveformmay be dispensed with by arranging for the transmitted power of thebase-station 6 to rise above the chirped waveform of the masking signal.In FIG. 11 we have a depiction of two bands being blocked. A lower bandappropriate for a first communications system is masked by chirpedwaveform 120 and a higher band corresponding to a second communicationssystem or standard is masked by the masking signal 121. The signal 122is that generated by the on-board base-station 6 and it will be seenthat it exceeds the power level of the masking signal 121.

The coupler network for such an embodiment is preferred to be such thatthe masking signal 120 is made diverse, the base-station signal 122 isnot diverse (since then only one base-station is required) and themasking signal 121 is thus also non-diverse.

To achieve this the coupling network 2 c is arranged to apply a firstlow band masking signal to the first antenna 3 from the first maskingsignal generator 2 a, and to apply a second low band masking signal tothe second antenna 4 from the second masking signal generator 2 b wherethe masking signals are diverse.

The coupling network 2 c also combines a signal transmitted from thebase-station 6 with a high band masking signal from one of the maskingsignal generators 2 a or 2 b and then applies the combined signal toboth the antennas 3 and 4.

The coupling network 2 c is also required to couple signals from themobiles being used in the aircraft cabin 5 from the antennas 3 and 4 tothe receiver section of the base-station 6.

One embodiment of the coupling network 2 c is shown in FIG. 12. Itcomprises a network of combiners 123 to 126, circulators 127 and 128,and duplexers 129 and 130. The duplexers 129 and 130 are formed of twocomplimentary low and high pass filters denoted a and b respectively. Inorder to provide a preferred degree of redundancy a backup switchingprocessor 131 is provided coupled to each of the masking signalgenerators 2 a, 2 b. This enhancement of the basic configuration will bedescribed later.

It will be seen that each of the masking signal generators 2 a, 2 bcomprise a first and second output for the low and high bandsrespectively. Considering the first low band outputs these are coupledto the low pass filters 129 a and 130 b of the diplexers and thencecoupled to the antennas 3 and 4. These signals are diverse.

It will be seen that the base-station 6 has a transmit output and areceive input. The masking signal generator 2 a high band masking signalis coupled to a combiner 123. This masking signal generator is normallyused to produce the masking signal but to cope with a potential failurebackup switching processor 131 may detect the failure and command theother masking signal generator 2 b to provide a high band maskingsignal. Thus, the other branch of the combiner 123 is connected to theother masking signal generator 2 b. The output of the combiner 123 isthe high band masking signal and it is passed to the combiner 124.Another input of the combiner 124 is coupled to the transmit output ofthe base-station 6. The combined signal is passed to a divider 126 todivide the signal. Each division of the signal is passed to a respectivecirculator 127, 128 and then passed via the high pass filters 130 a and129 b to respective antennas 3 and 4. Thus, a combination of thebase-station transmitter signal and the high band masking signal istransmitted into the cabin 5. This signal is not diverse in the sensethat the transmitted waveform is identical across the two antennas insuch a way that destructive interference is possible

The received signals are coupled from the antennas 3, 4 via the highpass filters 129 b and 130 a to the circulators 127, 128. These couplethe received signals to the combiner 125 which provides a combinedoutput to the received signal input of the base-station 6.

FIG. 13 shows a similar arrangement to cater for three bands 1, 2 and 3In this case the base-station 6 transmits on band 2 which is the middleband. (The diplexers are replaced with triplexers in this embodiment.)

FIG. 14 shows an arrangement which provides a notched masking signal. Inthis case the base-station transmitter signal is divided by a dividerbefore being combined with the masking signals in the middle band. Thecombined signals are diverse and coupled to the antennas via circulatorsand the triplexers to the antennas 3 and 4.

In the description of the use of the Fourier transform to create thehole or notch in the masking spectrum, it should be noted that othertransform and frequency nulling methods may be used which transform thesignal into spectral components to allow the pertinent components to becancelled before the inverse transform is applied.

It may also be possible to focus the available processing power to onlythe frequencies of the notch. An alternative method of generating thenotch operates by producing a copy of the normal chirped waveform thatis limited in bandwidth to the range of frequencies to be notched andsubtracting it from the original normal chirped waveform. Because thewaveform is repetitive, the spectrum of this signal will consist ofdiscrete lines with frequencies that are a multiple of the reciprocal ofthe total period of the waveform. Once the band to be notched has beendetermined in dependence upon the operating frequency of the basestation, all of the spectral line components to be nulled can bedetermined. The complex amplitudes of each of these components can bedetermined by performing the fourier summation according to theapplicable frequency. Once all of the complex amplitudes over thebandwidth to be nulled have been determined, the relevant complexsinusoids with the correct amplitudes, start phases and frequencies canbe computed and subtracted from the normal chirped waveform in order togenerate the notched waveform.

In a further improved method, the entire spectrum of the normal chirpedwaveform can be stored in a further look up table of complex weights.When it is desired to produce a notch in a particular band, the alreadyavailable required weights can be selected out of those stored. Thesecan then be used to generate the relevant complex sinusoids with thecorrect amplitudes, start phases and frequencies and subtract them fromthe normal chirped waveform in order to generate the notched waveform.

1. Connection inhibition apparatus to prevent mobile communicationequipment in a first zone connecting to a base-station in a second zoneoutside the first zone, the apparatus comprising: means to transmit amasking signal at a frequency or frequencies of the base-station withinthe first zone to mask transmissions from the second zone to inhibitconnection thereto wherein the masking signal is at least one chirpedwaveform.
 2. Apparatus as claimed in claim 1, wherein the masking signalis a windowed chirped waveform.
 3. Apparatus as claimed in claim 2,wherein the windowed chirped waveform is phase modulated.
 4. Apparatusas claimed in claim 1, wherein the masking signal is a chirped waveformmodulated by a randomising sequence.
 5. Apparatus as claimed in claim 4,wherein the randomising sequence is an M sequence.
 6. Apparatus asclaimed in claim 1, wherein the means to transmit a masking signaltransmits a plurality of masking signals arranged to be mutuallynon-coherent in order to prevent fading in the first zone.
 7. Apparatusas claimed in claim 1, wherein the means to transmit a masking signaltransmits over a spectrum of frequencies.
 8. Apparatus as claimed inclaim 1, wherein the means to transmit a masking signal substantiallydoes not transmit at a frequency or frequencies allocated to abase-station serving the first zone.
 9. Apparatus as claimed in claim 8,wherein the means to transmit a masking signal transmits a spectrum offrequencies with a hole in the spectrum which coincides with theallocated frequency.
 10. Apparatus as claimed in claim 1, wherein themeans to transmit a masking signal transmits a train of chirpedwaveforms.
 11. Apparatus as claimed in claim 2, wherein the chirpmodulated waveform is further biphase modulated by the elements of abinary sequence.
 12. Apparatus as claimed in claim 11, wherein theperiod of an element of the binary sequence is an integer multiple ofthe period of a chirp used to form the chirped waveform.
 13. Apparatusas claimed in claim 12, wherein a time of transition of one element ofthe binary sequence to the next is arranged substantially to coincidewith a flyback period of the chirp.
 14. Apparatus as claimed in claim11, wherein the binary sequence is a repeated finite length binarysequence.
 15. Apparatus as claimed in claim 14, wherein the finitelength binary sequence is an M sequence.
 16. Apparatus as claimed inclaim 11, wherein the binary sequence is a random sequence. 17.Apparatus as claimed in claim 1, wherein the chirped waveform iswindowed by application of an amplitude windowing function. 18.Apparatus as claimed in claim 17, wherein the windowing function has araised cosine time function.
 19. Apparatus as claimed in claim 1,wherein the masking signal is derived by performing a transform on achirped waveform to provide spectral components, nulling spectralcomponents coinciding with the allocated frequency or frequencies andperforming an inverse transform to provide the masking signal spectrumwith a hole.
 20. Apparatus as claimed in claim 19, wherein the transformis a Fourier transform process.
 21. Apparatus as claimed in claim 1,wherein the masking signal is derived by performing a Fourier transformon a complete sequence of chirps modulated by one finite length binarysequence, nulling spectral components coinciding with the allocatedfrequency or frequencies and performing an inverse Fourier transform toprovide the masking signal.
 22. Apparatus as claimed in claim 21,wherein the finite length binary sequences that modulate the chirpedwaveform are arranged to operate with different mutual phases in atleast two signals.
 23. Apparatus as claimed in claim 22, wherein thefinite length binary sequences that modulate the chirped waveform aregenerated using different polynomials in M sequence generators. 24.Apparatus as claimed in claim 1, wherein the masking signal is held as aset of values in memory.
 25. Apparatus as claimed in claim 24, furthercomprising a digital to analogue converter to convert the values inmemory to an analogue form.
 26. Apparatus as claimed in claim 25,wherein the memory is a look-up table.
 27. Apparatus as claimed in claim26, wherein the look-up table is addressed by a counter.
 28. Apparatusas claimed in claim 1, further comprising means to determine a locationof the first zone, wherein the means to generate the masking signal isresponsive to a determined location to generate a masking signalappropriate to the second zone associated with the determined location.29. A method of inhibiting connection of mobile communication equipmentin a first zone to a base-station in a second zone outside the firstzone, comprising: transmitting a masking signal within the first zone ata frequency or frequencies of the base-station to mask transmissionsemanating therefrom to inhibit connection thereto wherein the maskingsignal is a chirped waveform.
 30. A method as claimed in claim 29,wherein the masking signal has wide bandwidth.
 31. A method as claimedin claim 30, further comprising providing a wide bandwidth signal with ahole or holes in its spectrum coinciding with a frequency or frequenciesallocated to a base-station or base-stations serving the first zone. 32.A method as claimed in claim 30, wherein the wide bandwidth signal isderived from a chirp.
 33. A method as claimed in claim 29, wherein themasking signal is formed from at least two signals of randomly varyingphase relationship.
 34. A method of providing a windowed chirp with ahole in its spectrum for use in connection inhibiting apparatus asclaimed in claim 1, comprising: forming a chirp; applying an amplitudewindow function to the chirp to provide a windowed chirp performing aFourier transform to the windowed chirp to provide a transformedwindowed chirp; nulling spectral components of the transformed windowedchirp co-inciding with frequencies allocated for use by a base-stationserving the first zone; and performing an inverse Fourier transform toprovide a windowed chirp with a hole.
 35. A method as claimed in claim34, further comprising forming a masking signal that includes applying arandomising sequence to a chirped waveform.
 36. A method as claimed inclaim 35, wherein the randomising sequence has a finite length.
 37. Amethod as claimed in claim 36, wherein the randomising sequence hasinfinite length.
 38. A method as claimed in claim 35, wherein therandomising sequence is an M sequence.
 39. A method as claimed in claim34, wherein a location is determined and a masking signal is generatedbased on the determined location.
 40. Connection inhibition apparatus toprevent mobile communication equipment in a first zone connecting to abase-station in a second zone outside the first zone, the apparatuscomprising: means to transmit a masking signal within the first zone tomask transmissions from the second zone wherein the masking signalcomprises a spectrum having a hole at a frequency at which abase-station in the first zone operates such that connection tobase-stations in the second zone are inhibited and that connection to abase-station in the first zone are permitted.
 41. Connection inhibitionapparatus to prevent mobile communication equipment in a first zoneconnecting to a base-station in a second zone outside the first zone,the apparatus comprising: means to transmit a masking signal at thefrequency or frequencies of the base-station within the first zone tomask transmissions from the second zone to inhibit connection theretowherein the masking signal comprises two or more mutually non-coherentsignals mutually non-coherent in order to prevent fading in the firstzone.
 42. Apparatus as claimed in claim 40, wherein the masking signalcomprises two or more mutually non-coherent signals modulated byrandomising sequences.
 43. Apparatus as claimed in claim 41, wherein themasking signal comprises two or more mutually non-coherent signalsmodulated by randomising sequences.
 44. Apparatus as claimed in claim42, wherein the randomising sequence is a finite length randomisingsequence.
 45. Apparatus as claimed in claim 43, wherein the randomisingsequence is an infinite length randomising sequence.
 46. Apparatus asclaimed in claim 44, wherein the randomising sequence is an M sequence.47. Apparatus as claimed in claim 45, wherein the randomising sequenceis an M sequence.
 48. Apparatus as claimed in claim 43, wherein themasking signal comprises a chirped waveform.
 49. A coupling network forcoupling a first and a second masking signal from a first masking signalsource, a third masking signal from a second masking signal source whichsignal being diverse relative to the first masking signal and atransmitter signal to a first and a second antenna, comprising: means tocouple the first and the third masking signals to respective ones of thefirst and the second antenna; means to combine the transmitter signaland the second masking signal to provide a first combined signal; andmeans to couple the combined signal to both antennas.
 50. Apparatus asclaimed in claim 49, wherein the second masking signal source provides afourth masking signal diverse to the second masking signal and whereinthe masking signals include a notch at the frequency of the transmittersignal and further comprising a combiner to combine the transmittedsignal to both the second and the fourth masking signals to providefirst and second combined signals and means to couple the first andsecond combined signals to respective ones of the first and secondantennas.
 51. A method for producing a notched chirped waveform for useas a masking signal comprising the steps of: providing a chirpedwaveform; identifying a part of the chirped waveform where a notch isrequired; applying a transform to the identified part to providespectral components; deriving from the spectral components those thatare to be nulled; applying an inverse transform to the spectralcomponents to be nulled; and subtracting the inverse transformedspectral components from the chirped waveform to generate the notchedchirped waveform.
 52. A method as claimed in claim 51, wherein thechirped waveform is held in memory.
 53. A method as claimed in claim 52,wherein the spectrum of the chirped waveform is also held in memory as aset of complex weights and further comprising the steps of: selectingfrom the set of complex weights a selection applicable to a desirednotch; deriving from the selection a set of waveforms; and applying theset of waveforms to the chirped waveform to generate a notched chirpedwaveform.